This invention relates to a camera and, more particularly, to one in which a current effect mode liquid crystal cell is used as a focusing screen.
The viewfinder of a camera can present a variety of information to the user, including an indication of the photographing composition, focusing information, data indicative of photographing conditions, etc., and various methods to fulfill such roles have been proposed. Focusing methods are generally divided broadly into the two categories of a matte surface focusing screen and a transparent focusing screen. In the former, the light-diffusing nature of the surface of ground glass or other material is utilized and the viewfinder image of the subject to be photographed becomes dim with the reduction in light intensity on the diffusion surface, resulting in focusing difficulties and interfering with decisions in respect of composition.
As for the transparent focusing screen, several arrangements including a split-image method, a microprism method and a cross-hair method are currently utilized. In these arrangements, focusing is possible only on a specific part (usually the central area of the field of view) of the finder; on the remainder of the field of view, differences between the in-focus and out-of-focus states are unclear. In addition, a shadow tends appears on the focusing part or area of the screen as the lens is stopped down, whereby focusing accuracy falls.
In order to provide in the finder an indication of photographing conditions, such elements as a meter indicator, a lamp, an LED and an indication plate have been arranged near the focusing screen, on the pentagonal prism surface and in the clearance space between the focusing screen and the pentagonal prism. This permits the f-stop value, the shutter speed, blur and/or exposure information and warnings of low-high-luminance to be indicated enabling more clear and properly exposed photographs to be produced. Each of these known methods, however, has required a complicated indication system.
Presently, liquid crystals are divided into two types--one is the electric field effect mode liquid crystal that works with only a dielectric torque produced by an interaction between a dielectric anisotrophy of the liquid crystal molecules and an electric field, while the other is the current effect mode liquid crystal that operates in accordance with both an electrical conduction torque based upon an electrical conductivity anisotrophy of the liquid crystal molecules and the aforesaid dielectric torque. An electric field effect mode liquid crystal is further classified into TN, DAP, HAN, PT and GH modes according to an orientation characteristic of the liquid crystal molecules by a dielectric torque. In liquid crystals of the TN, DAP and HAN modes, an indication is made with an attached polarizing plate and utilizing rotary polarization of the liquid crystal molecule orientation. In a GH mode liquid crystal, an indication is made by light absorption of dyes added to the crystal in very small amounts. In a PT mode liquid crystal, an indication is made (similarly to a current effect mode liquid crystal) by light dispersion and this light dispersion occurs because the screw axis of the cholesteric liquid crystal upon which an electric field is impressed is not uniformly perpendicular to the electrode and becomes a group with an inclination. However, a liquid crystal with a screw axis of a certain pitch absorbs visible rays and it is therefore necessary to use for the focusing screen liquid crystals having a screw axis with a pitch on the order to several um that does not absorb visible rays. Furthermore, the driving voltage of liquid crystals of this mode must be more than 20-25 volts which is incompatible with CMOS circuits that are most suitable for driving liquid crystals. It accordingly becomes necessary for the camera having batteries as a power source to include a built-in voltage boosting circuit which, as a practical matter, is a serious drawback as compared to a DSM (Dynamic Scattering Mode or DS mode) liquid crystal, as hereafter explained.
On the other hand, only one type of DS mode is given as a current effect mode and its indication states are determined by the effect of light dispersion.
Thus, various types of liquid crystal indicators have been proposed according to the orientation of their molecules, but only two types--TN mode in an electric field effect crystal and DS mode in a current effect crystal--have been put to practical use and are on the market, while other modes are still in the research phase of development. A current effect mode liquid crystal is represented by an Nn liquid crystal having a moderate ion nature and when a sufficiently large electric field is impressed thereon, current flows in the liquid crystal; the crystal itself thereby enters a turbulent flow state with the action of both a dielectric torque and an electrical conduction torque such that light is intensively dispersed and an indication is made by the difference in contrast between the portion where the electric field is impressed and the portion where no such field is impressed. A current effect mode liquid crystal is so constructed that a pair of NESA-glass plates provided with a transparent electrode are held at a clearance of about 10 um and an appropriate liquid crystal material (such as a schiff compound or an azo-compound) is enclosed therebetween. It is also possible to add additives thereto to promote a turbulent flow state.
This turbulent flow state is a flow of liquid crystal molecule groups of about 10 um in size. Thus, when used as a matte surface of the camera focusing screen and held between two pieces of flat and transparent NESA-glass, the liquid crystal advantageously displays its characteristics.